SUMMARY While current antiretroviral therapies (ART) are able to control HIV replication, they are unable to fully restore health or a normal immune status. ART-treated individuals still experience several co-morbidities including increased cardiovascular disease, bone disorders, and cognitive impairment. Most importantly, therapy interruption leads to the re-emergence of viral replication and progression to AIDS. Therefore, new approaches aimed at eradicating or functionally curing HIV infection are desperately needed. An under-studied strategy to eliminate latently infected cells after viral reactivation relies on the ability of immune cells to mediate antibody- dependent cellular cytotoxicity (ADCC). The RV144 HIV-1 vaccine trial in Thailand elicited a 31.2% protective efficacy, making it the first vaccine trial with any level of success in generating a protective response. Subsequent analyses indicated that this modest protection was correlated with the generation of antibodies (Abs) with high ADCC activity, in the presence of low plasma IgA Env-specific Abs. This suggests that ADCC may have contributed to the protection observed in the RV144 trial. But key unanswered questions exist that prevent researchers from specifically triggering the ADCC response with novel treatments or immunogens: Why did the RV144 trial generate such a strong ADCC response? What about the CRF01_AE subtype of HIV-1, which predominates the Thai AIDS epidemic, might make it especially susceptible to ADCC? Does Env conformation affect ADCC responses? Answering these questions will prove crucial to the design of improved strategies to eliminate HIV-1-infected cells. The long-term goal of the research described in this proposal is to inform the development of new strategies for utilizing the ADCC response to eradicate the HIV-1 infection. To achieve this goal, we will begin by describing in molecular detail the Env conformations that are susceptible to attack by Abs that induce ADCC, and to determine the structural elements of Env from distinct HIV-1 strains that mediate transition to these conformations. Our central hypothesis is that Env has intrinsic access to downstream conformations that are recognized by easily-elicited non-neutralizing Abs. Some of which, like the anti-cluster A Abs, have potent ADCC activity. In support of this hypothesis, we recently demonstrated using Ab-binding assays, cryo-electron microscopy (Cryo-EM), and single-molecule Frster resonance energy transfer (smFRET) imaging that HIV-1 Env can adopt a conformation that is sensitive to attack by Abs that have potent ADCC activity (State 2A). The rationale underlying this proposal is that characterization of the structure of Env State 2A, as wells as other conformations recognized by non-neutralizing Abs, and the elements that mediate stabilization of these conformations will inform new strategies to eliminate the latent HIV reservoir.